Vacuum closure device

ABSTRACT

This invention relates to the design of tissue covering elements for use in vacuum assisted tissue apposition systems, wherein the geometry of the covering elements favors the application of contractile forces over compressive or extensive forces at the tissue interface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Phase of the PCT InternationalApplication No. PCT/GB2009/000696 filed on Mar. 13, 2009, designatingthe United States and published on Sep. 17, 2009 as WO 2009/112848, andwhich claims priority to Great Britain Patent Application No. 0804654.2,filed Mar. 13, 2008. The disclosure of these prior applications isincorporated by reference in their entirety and should be considered apart of this specification.

FIELD OF THE INVENTION

This invention relates to the design of tissue covering elements for usein vacuum assisted tissue apposition systems, wherein the geometry ofthe covering elements favours the application of contractile forces overcompressive or extensive forces at the tissue interface.

BACKGROUND OF THE INVENTION

Topical negative pressure (TNP) therapy has rapidly grown to be ofexcellent utility in the medical field, particularly in the treatment ofwounds.

A number of the current negative pressure systems available involve theapplication of a porous, deformable wound filler to the wound. The basicprinciple of TNP therapy is to create a closed cavity over the wounditself by means of a thin, flexible sealing film adhered to thepatient's sound skin surrounding the wound; admitting one end of anaspirant conduit into the closed cavity, the conduit being sealed to theflexible film, for example; and connecting a distal end of the aspirantconduit to a vacuum source such as an electrically driven vacuum pump,for example, to create a pressure lower than the surrounding ambientatmospheric pressure within the wound cavity. As is known to the skilledperson the lower pressure creates many beneficial therapeutic effects onthe wound including increased blood flow to the wound and fastergranulation of tissue, for example. When the vacuum pump is switched on,the adjacent surfaces formed cavity expand into the wound filler,compressing it up to the point where it can mechanically resist furtherdeformation. In this state, it is hypothesised that, in a wound cavity,both compressive and extensive forces are exerted on the micro scale atthe tissue surface while extensive forces are exerted on the macro scalea short distance from the tissue-filler interface. The extent of thesecompressive and extensive forces is determined by the applied (negative)pressure, the mechanical properties of the surrounding tissue, fillerand drape and the geometry of the wound.

One TNP system provide the user with sheets of foam of varying geometrythat are routinely cut to shape at the site of application to conform tothe surface of the wound or fill the wound cavity. In this regard, forsome applications, including those targeted here, this technique issub-optimal. The problem here is that even if we assume uniform foam,drape and tissue mechanics across the patient population, the woundgeometry will vary significantly from patient to patient. FIG. 1demonstrates the effect of applying standard pressure and mechanics(tissue, drape and foam) to varying geometries of application; the forcevectors generated vary widely. For most applications, particularly tocavity wounds, the extent of this variation is not great and does notaffect the efficacy of the treatment significantly: surrounding tissueis generally expanded in the desired direction, towards the centre lineof the cavity volume (see FIG. 1). However, for shallow wounds orincisional wounds, the desired mechanical forces are not afforded by thecurrent method; in general, a compressive force perpendicular to thetissue surface is generated with a minor force generated in thedirection parallel to the wound surface. For shallow wounds and incisionwounds, it is desirable to generate significant forces parallel to thewound surface in the direction of wound closure in the same way as it isdesirable to generate this arrangement in wound cavities by thetraditional method.

We are not aware of topical negative pressure devices capable ofgenerating significant forces parallel to largely two-dimensionalsurfaces of attachment.

SUMMARY OF SOME EXEMPLIFYING EMBODIMENTS

The invention is concerned with the component of a topical or internalnegative therapy apparatus which forms the interface with the tissuesurface and provides a vacuum cavity above the tissue surface.Traditionally, in the field of topical negative pressure, this interfaceis achieved using a porous wound filler and an occlusive orsemi-occlusive adhesive drape (to create the substantially air tightseal), as described above.

This device is referred to herein as the tissue covering element.

The invention is not restricted to the means of generating negativepressure or regulating negative pressure or to the means of transmittingthe source of negative pressure to the site of application, but isdirected to the design geometry of the tissue covering element.

The comparable component of the present invention in conventionalnegative pressure systems generates very low contractile forces in theplane which is parallel to largely flat or convex tissue surfaces but,in contrast, generates relatively large compressive or extensive forcesin the plane which is perpendicular to the tissue surface. This isbecause the majority of the surface area of the vacuum cavity arrangedin these cases is parallel to the tissue surface and the forces aregenerated perpendicular to this.

The force vectors generated are predominantly compressive forces. Suchforces are appropriate for the closure of cavity wounds but are contraryto the desire for contraction of the wound margin in the direction ofclosure for largely flat or convex defects. As FIGS. 1 b and 1 c furtherillustrate, the generation of significant forces parallel to the tissuesurface cannot be achieved easily when the wound covering element islargely parallel to the tissue surface.

The present invention concerns a tissue covering element that forms aninterface with largely flat or convex tissue surface geometries togenerate a vacuum cavity having a geometry that is predominantlynon-parallel to the tissue surface and as a consequence generatessignificant forces parallel to the tissue surface when the cavity isplaced under reduced pressure. Preferably, the forces generated are inthe direction of wound closure.

It is recognised here that the ‘direction of wound closure’ should notonly consider the geometry of the wound surface but also the mechanicalproperties of the surrounding tissue (e.g. Langer's lines) that mayinfluence the most desirable geometry of closure forces.

It should be further noted that the geometry does not exclude thegeneration of compressive or extensive forces in the plane perpendicularto the tissue surface.

Thus, according to a first aspect of the invention there is provided atissue covering element for use in a vacuum assisted closure system, thetissue covering element comprising:

-   -   i) a wound contacting surface, said surface being positionable        on either side of a wound margin;    -   ii) a bridging portion which bridges the wound and in use        provides at least a partial vacuum above the wound, and wherein        the bridging portion comprises a higher ratio of internal        surfaces that are aligned substantially non-parallel relative to        the wounded tissue surface, to: surfaces that are aligned        substantially parallel relative to the wounded contacting        surface.

According to a second aspect of the invention there is provided a methodof closing a wound, said method comprising;

-   -   i) locating a tissue covering element according to the invention        at a wound site such that the bridging element bridges the wound        margins;    -   ii) providing a device for generating a negative pressure;    -   iii) applying a negative pressure to the tissue covering element        in a manner such that the bridging portion of the tissue        covering element creates at least a partial vacuum cavity above        the wound.

A method of applying a contractile force to a wound, said methodcomprising;

-   -   i) locating a tissue covering element according the invention at        a wound site, such that the bridging element bridges the wound        margins;    -   ii) providing a device for generating a negative pressure;    -   iii) applying a negative pressure to the tissue covering element        in a manner such that the bridging portion of the tissue        covering element creates at least a partial vacuum cavity above        the wound.

According to a further aspect of the invention there is provided the useof a device according to the present invention in a vacuum assistedclosure apparatus.

According to a further aspect of the invention there is provided the useof a device according to the present invention in wound therapy.

According to a further aspect of the invention there is provided thedevice or use of the device according to the present invention as hereindescribed with reference to the accompanying Examples and Figures.

The contractile forces so generated, parallel to the tissue surface aretransmitted to the tissue margin and this tissue experiences a forcedirected approximately towards the centre of the vacuum cavity.

The tissue surfaces can be largely flat or convex.

The term ‘largely flat’ is herein taken to mean surfaces with verticaldimensions no greater than 20% of the shortest of the other dimensions(FIG. 2).

The tissue surfaces can be internal or external to the body.

The tissue surfaces can be on soft tissues (e.g skin, cartilage, tendon,ligament, muscle, fascia) or hard tissues (e.g bone)

In embodiments of the invention such a surface geometry can be affordedby the use of corrugated or concertina-folded structures. Thecorrugation of the vacuum cavity cannot easily or painlessly be achievedin the tissue surface but can easily be achieved in the tissue coveringelement by providing corrugations that extend in the direction largelyperpendicular to the tissue surface. It is important that this geometryis not completely destroyed under the influence of reduced pressurewithin the vacuum cavity, although some geometrical distortion isunavoidable.

It is important that the covering element is sufficiently flexible toallow good conformability across the tissue but also sufficiently rigidto maintain a higher ratio of surfaces perpendicular to the tissuesurface than those parallel to the tissue surface. This combination offlexibility and rigidity can be achieved most simply by combiningrelatively inflexible surfaces largely perpendicular to the tissuesurface with flexible hinges.

The tissue covering element of the present invention is defined ashaving at least 50% of its internal surface area distribution at anangle of 45° or greater relative to the underlying, largely flat orconvex tissue surface. The net force is contractile.

In further specific embodiments of the invention the tissue coveringelement is defined as having at least 50% of its internal surface areadistribution at an angle of 80° or greater relative to the underlying,largely flat or convex tissue surface. The net force is contractile.

There may be some medical applications in which it may be desired toprovide a wound covering element that expands rather than contracts thewound. In such instances the tissue covering element defined as havingthe majority of its internal surface area distribution at an angle ofbetween about 5° to 45° relative to the underlying, largely flat orconvex tissue surface.

The shortest distance between the margins of the wound is defined as X,and the length of the bridging element is defined as Y. In embodimentsof the invention Y is at least 110% of X. In further embodiments of theinvention Y is at least 141% of X, as illustrated in FIG. 5. The tissuecovering element can be defined in any single dimension as one with aninternal surface length not exceeding 1000% of the shortest line betweenopposing points on the tissue margin (FIG. 5). The additional surfacelength is used to generate corrugations or folds that increase thesurface area of the covering non-parallel to the tissue surface with theaim of maximising the perpendicularity of the covering to the tissuesurface.

For application to largely one-dimensional incision wounds, an inverted‘V’ structure, as illustrated in FIG. 3 c, is an advantageous geometry.

For application to largely flat circular wounds in locations ofisotropic skin tension, a corrugated concentric ring structure, asillustrated in FIG. 7, is an advantageous geometry.

For application to large flat wounds in locations of anisotropic skintension, a multiple inverted ‘V’ ‘concertina configuration in desirable.The lines of the concertina folds being positioned parallel to localLanger's lines to effect wound closure (FIG. 8).

For example, for application to convex surfaces, such as the extremitiesof the body or the bones of the body, a cylindrical concertinaconfiguration is desirable (similar to a shock absorber ‘boot’). Thelines of the concertina folds being perpendicular to the axis of thelimb (FIG. 9) for application to bone, or parallel to local Langer'slines to effect topical wound closure (FIG. 8).

In practical use, to avoid the eventuality where the application ofnegative pressure to the cavity causes gross distortion of the desiredgeometry of the covering, the two-dimensional surfaces of the device areconstructed of relatively stiff materials in comparison to the foldedregions, which should flex easily. That is not to imply the use of rigidmaterials, uncomfortable for the wearer.

The tissue covering element must be attached to the patient in a mannerthat creates an substantially air tight seal of a quality that allowsthe vacuum source to maintain the target vacuum level within theenclosed volume. Practically, attachment to the patient can be achievedby means of any bonding mechanism. Preferably attachment is achieved byusing an adhesive, such as an acrylate- or silicone-based adhesivecommonly used for the attachment of medical devices and well known inthe art. It is also possible that attachment can be achieved byutilising the pressure differential alone, in the same manner as asuction cup. However, this is difficult to achieve in reality due to themechanical properties of tissue. Connection of the enclosed volume tothe vacuum source can be achieved by any means obvious to a personskilled in the art, for example via a central penetrating port or inbetween the covering element and the bonded perimeter. Preferably, thevacuum source is connected to the enclosed volume by a port penetratingthe surface of the covering element. It is additionally beneficial forthe coupling of the vacuum source to the covering element to be achievedby a reversible means to facilitate repeated connection anddisconnection over the duration of dressing wear. It is also preferablethat connection and disconnection can be easily achieved by the patientand applier.

The invention is not restricted to filling elements that may optionallybe positioned within the vacuum enclosure.

The devices so described can be applied in a range of medicalapplications where generation of forces parallel to a surface ofattachment is desirable for example, the joining of tissue and bonelesions or defects. Both topical and internal applications can beforeseen, from the application of contractive force to bone breaks tothe closure of surface wounds, including surgical incisions.

The invention will now be described, for illustrative purposes only,with reference to the accompanying Examples and Figures, wherein theFigures illustrate:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Force vectors generated within hypothetical tissue cavities.

(a) Force vectors generated by a conventional tissue covering elementwhen applied to (a) a cavity wound (b,c) and to a largely flat or convexwound.

FIG. 2: Illustrates a substantially flat wound.

FIG. 3: Embodiments of the tissue covering element of the presentinvention.

FIG. 4: The distribution of surface area angles for any given tissuecovering element can be represented in a histogram.

FIG. 5: Determining the optimal dimension of the tissue coveringelement.

FIG. 6: Relationship between the dimension of the tissue coveringelement and the forces generated.

FIG. 7: A schematic of tissue covering element for application tolargely flat circular wounds in locations of isotropic skin tension.

FIG. 8: A schematic of tissue covering element for application tolargely flat wounds in locations of anisotropic skin tension.

FIG. 9: A schematic of tissue covering element for application to a limbor bone tissue defect.

DETAILED DESCRIPTION OF SOME EXEMPLIFYING EMBODIMENTS

FIG. 1 illustrates the forces generated by conventional tissue coveringelements when applied to (a) cavity tissue defect and (b,c)substantially flat or convex tissue defects.

-   (a) the tissue covering element 1 is applied to wound 2 within    tissue 3. This is a cavity tissue defect. The application of a    partial vacuum generates substantially equal compressive and    contractile forces. This is optimal for the healing mechanism of    this wound type.-   (b) the tissue covering element 1 is applied to wound 2 within    tissue 3. This is a shallow, largely flat tissue defect. The    application of a partial vacuum generates predominantly compressive    forces and minimal contractile forces. This is sub-optimal for the    healing mechanism of this wound type in which there is a desire for    contraction of the margin of the wound in the direction of the    closure.-   (c) the tissue covering element 1 is a molded cover which is applied    to wound 2 within tissue 3 to form a vacuum cavity above the wound.    The surfaces of the covering element are predominantly parallel to    the surface of the wound. The application of a partial vacuum    generates predominantly compressive forces and minimal contractile    forces. This is sub-optimal for the healing mechanism of this wound    type in which there is a desire for contraction of the margin of the    wound in the direction of the closure.

FIG. 2 illustrates a substantially flat tissue defect. Such a wound hassurfaces with vertical dimensions (z) no greater than 20% of theshortest of the other dimensions (x) and (y).

FIG. 3 illustrates various embodiments of the present invention. In (a)the tissue covering element 1 which is applied to a substantially flator convex tissue defect 3 comprises a plurality of corrugations whichgenerate a net force parallel to the tissue surface, thereby generatinga contractile force in the direction of closure of the wound. In (b) thetissue covering substantial net force is parallel to the tissue surface.In the embodiment illustrated in (c) the tissue covering element 2comprises a base member 4 a and 4 b which is applied to the tissuesurface 3. From this base member the walls 5 a and 5 b extend to theapex, thereby forming an inverted “V” shaped cavity above the surgicalincision. The base member 4 a and 4 b and the walls 5 a and 5 b are madeof a relatively inflexible material, with a flexible hinge 6 a 6 bforming the junction between the base member 4 a and 4 b and the walls 5a and 5 b and a further flexible hinge 6 c provided along the apex ofthe “V”. A vacuum connection port 7 is provided on one of the wallelements.

The distribution of surface area angles for any given tissue coveringelement can be represented in a histogram. FIG. 4 illustrates this forthe following tissue covering element designs:

-   -   (a) a conventional negative pressure cover geometry    -   (b) a cover geometry with 30° surface incidence angle    -   (c) a cover geometry with 45° surface incidence angle    -   (d) a hemispherical cover geometry    -   (e) a cover geometry with 90° surface incidence angle    -   (f) a cover geometry with 90° surface incidence angle (larger        surface area than in (e)

FIG. 5 illustrates the optimal geometry of the tissue covering elementrelative to the tissue defect. The tissue defect has a desired axis ofcontraction between points A and B. The dimension of the tissue coveringelement aligned between A and B is preferably longer than the directlength labelled X.

FIG. 6 illustrates that for any perimeter length the force generatedperpendicular to a flat surface of attachment is constant. It can alsobe seen that forces generated parallel to the flat surface (contractile)scale directly in proportion to the vertical height of the corrugationdivided by half its period length.Force=Pressure×Area

In the example illustrated in FIG. 6, let perimeter length x be directlyproportional to surface area y (true for simple designs pictured inFIGS. 6 and 9). For a constant vacuum pressure p, the resolved forcesgenerated from the surfaces shown in FIG. 6 a are shown in FIG. 6 b.

FIG. 7 is a schematic of a tissue covering element for application tolargely flat circular wounds in locations of isotropic skin tension. Thetissue covering element is a corrugated concentric ring structure. Thecentre of the element may be convex or concave when viewed from above.

FIG. 8 is a schematic of a tissue covering element for application tolargely flat circular wounds in locations of anisotropic skin tension.The tissue covering element is a multiple inverted ‘V’ concertinaconfiguration. The lines of the concertina folds being positionedparallel to local Langer's lines to effect wound closure.

The tissue covering element 11 is substantially square. An adhesive 12forms a peripheral border around the tissue contacting surface of thetissue covering element. The element 11 further comprises a bridgingelement which is formed of a plurality of separated inverted V-shapedelements 13. The V-shaped elements are hinged, to allow greaterflexibility. First 14 a and second (not shown) hinges are provided atthe join between the inverted V-shaped element and the upper surface ofthe tissue contacting surface (ie the surface that faces upwards awayfrom the tissue). A third hinge 14 c is provided at the apex of theinverted “V”. The arrows “X” illustrate the direction of local Langers'lines. The arrows “Y” illustrate the direction of contraction generatedunder vacuum.

For application to convex surfaces, such as the extremities of the bodyor the bones of the body, a tissue covering element 21 having acylindrical concertina configuration is desirable (similar to a shockabsorber ‘boot’) as illustrated in FIG. 9. The proximal 21 a and distal21 b ends of the tissue covering element are fixed to the limb or bone22 (defect positioned under the tissue covering element). The lines ofthe concertina folds being perpendicular to the longitudinal axis of thelimb 25. The arrow “Y” illustrates the direction of contractiongenerated under vacuum.

EXAMPLES Example 1 Construction of the Incision Closure Device Picturedin FIG. 3 c

A device of the design pictured in FIG. 6 was moulded using atransparent, heat-curable medical grade silicone elastomer. The devicehad a dome-profiled pressure cracking valve (Minivalve International B.V.) cast into one of it's cavity faces. The flat surfaces of theinverted ‘V’ section of the device was reinforced with pre-curedmechanically stiff silicone elastomer.

Example 2 Incision Closure with the Device of Example 1

The device prepared in Example 1 was positioned over a gaping linearincision made into a porcine belly cadaver. Partial vacuum was appliedto the device via the crack-valve port. A pressure of −100 mmHg wasachieved relative to ambient atmospheric pressure (660 mmHg absolutepressure). The device deformed by hinging about the highest point of theinverted ‘V’ section, causing contraction of the tissue around theincision in a direction perpendicular to it, thus achieving closure ofthe wound.

Example 3 Construction of Device for the Closure of Open Area Wounds inLanger's Line Neutral Locations

A design of the concept pictured in FIG. 7 was moulded using aheat-curable medical grade silicone elastomer. The mould used was acollapsible funnel (Normann, Copenhagen) in the collapsed position. Whenthe elastomer was cured, the concentric finned device was removed byopening the funnel.

Example 4 Radial Contraction of Tissue with the Device of Example 3

The device prepared in Example 3 was modified with a central luer lockfitting and connected to a partial vacuum of −100 mmHg relative toambient atmospheric pressure. The device was positioned on a livingperson's abdomen and allowed to seal. The device corrugated under thereduced internal pressure and exerted a radial contractile force on theadjoining tissue in the direction of the centre of the device. Tissuewas contracted by approximately 15% of the original device diameter.

What is claimed is:
 1. A tissue covering element for use in a vacuumassisted closure system, the tissue covering element comprising: atissue contacting portion, said portion being positionable on eitherside of a wound margin of a wounded tissue surface; and a bridgingportion which bridges the wound and in use provides at least a partialvacuum above the wound, and wherein the bridging portion when in usecomprises a higher ratio of internal surfaces that are alignedsubstantially non-parallel relative to a skin surface compared tosurfaces that are aligned substantially parallel relative to the skinsurface such that a contractability of at least a portion of the tissuecontacting portion toward a middle portion of the wound is greater inthe direction parallel to the skin surface than the contractability ofthe wound cover in the direction that is substantially non-parallelrelative to the skin surface when vacuum is applied above the wound, andwherein the bridging portion is configured for anisotropic collapseparallel to the skin surface.
 2. A tissue covering element according toclaim 1, wherein the shortest distance between the margins of the woundis defined as X, and said length of the bridging element is defined asY, wherein Y is at least 110% of X.
 3. A tissue covering elementaccording to claim 2, wherein Y is at least 141% of X.
 4. A tissuecovering element according to claim 1, wherein at least 50% of theinternal surface area distribution of the bridging element is at anangle of at least 45° relative to the wounded tissue surface.
 5. Atissue covering element according to claim 1, wherein at least 50% ofthe internal surface area distribution of the bridging element is at anangle of at least 80° relative to the wounded tissue surface.
 6. Atissue covering element according to claim 1, wherein a hinge isprovided between the bridging element and the tissue contacting portion.7. A tissue covering element according to claim 1, wherein the tissuecovering element comprises at least two bridging elements.
 8. A tissuecovering element according to claim 1, wherein the bridging elementcomprises an inverted V geometry.
 9. A tissue covering element accordingto claim 8, wherein a further hinge is provided at the apex of theinverted V.
 10. A method of using the tissue covering element of claim 1in a vacuum assisted wound closure device, comprising: positioning thetissue covering element over a wound; providing a source of reducedpressure to a space between the tissue covering element and the wound;contracting one or more sides of the wound toward a middle portion ofthe wound.
 11. A tissue covering element according to claim 1, whereinthe tissue covering element is configured to exert a contractile forceon the wound configured to draw wound margins on either side of thewound together.
 12. A tissue covering element according to claim 1,wherein the bridging portion comprises a plurality of walls joined byhinges.
 13. A tissue covering element according to claim 12, wherein thewalls are relatively inflexible and the hinges are relatively flexible.14. A tissue covering element according to claim 12, wherein theplurality of walls are configured to be perpendicular to the skinsurface when collapsed.
 15. A method of closing a wound, said methodcomprising; locating a tissue covering element at a wound site withwound margins, such that a bridging portion of the tissue coveringelement bridges the wound margins; applying a negative pressure to thetissue covering element, wherein the bridging portion under negativepressure exerts a contractile force to at least a wound contactingsurface of the tissue covering element on either side of the woundmargins, drawing the wound contacting surface of the tissue coveringelement on either side of the wound margins toward a center of thewound, wherein the bridging portion contracts more in a directionparallel to a skin surface surrounding the wound than in a directionperpendicular to the skin surface and contracts anisotropically parallelto the skin surface.
 16. A method of closing a wound according to claim15, wherein the bridging portion is placed above the skin surface.
 17. Amethod of closing a wound according to claim 15, wherein the tissuecovering element forms a vacuum cavity over the wound.
 18. A method ofclosing a wound according to claim 15, wherein the bridging portioncomprises a plurality of walls joined by hinges.
 19. A tissue coveringelement according to claim 18, wherein the walls are relativelyinflexible and the hinges are relatively flexible.
 20. A tissue coveringelement according to claim 18, wherein the plurality of walls areperpendicular to the skin surface when collapsed.
 21. A negativepressure wound therapy system, comprising: a tissue covering elementadapted to cover a wound and to maintain reduced pressure between thetissue covering element and the wound, the tissue covering elementcomprising: a cover layer configured to form a cavity over the woundwhen a peripheral portion of the cover layer is positioned around thewound; and a plurality of adjacent, parallel corrugations, thecorrugations extending in a direction such that the corrugationsanisotropically contract in a direction parallel to a wound surface whenreduced pressure is applied between the tissue covering element and thewound in use, thereby causing one or more edge portions of the coverlayer to contract toward a middle portion of the wound.
 22. A negativepressure wound therapy system according to claim 21, wherein eachcorrugation has a triangular shape.
 23. A negative pressure woundtherapy system according to claim 21, wherein the corrugations each hasan inverted V shape.
 24. A negative pressure wound therapy systemaccording to claim 21, wherein the corrugations are configured tocontract when reduced pressure is applied to the cavity.
 25. A negativepressure wound therapy system according to claim 21, wherein thecorrugations are configured to exert a contractile force on the woundwhen reduced pressure is applied to the cavity.
 26. A negative pressurewound therapy system according to claim 21, further comprising a sourceof reduced pressure.
 27. A method of exerting a closing force on awound, said method comprising; positioning a tissue covering elementover the wound so as to create a substantially sealed space between thetissue covering element and the wound; reducing the pressure in thesubstantially sealed space between the tissue covering element and thewound, thereby contracting the tissue covering element in a first lineardirection that is toward a center or a centerline of the tissue coveringelement and parallel to the wound surface, wherein the tissue coveringelement contracts more in a first linear direction that is parallel tothe wound surface than in a second linear direction that is parallel tothe wound surface, the second linear direction being perpendicular tothe first linear direction, and thereby exerting a net contractile forceon the wound in the first direction.
 28. A method of closing a woundaccording to claim 27, comprising contracting the tissue coveringelement in the first direction that is toward the center or centerlineof the tissue covering element more than in a second direction that istransverse to the first direction, thereby contracting the wound in saidfirst direction more than in said second direction.
 29. A method ofclosing a wound according to claim 27, wherein contracting the tissuecovering element in the first direction that is toward the center orcenterline of the tissue covering element comprises contracting one ormore folded structures in the first direction that is toward the centeror centerline of the tissue covering element.
 30. A method of closing awound according to claim 27, wherein contracting the tissue coveringelement in the first direction that is toward the center or centerlineof the tissue covering element comprises contracting one or morecorrugations formed in the tissue covering element.